A lithium iron phosphate positive electrode material, a preparation method thereof, a lithium ion battery and an electric device

Abstract

The application provides a lithium iron phosphate positive electrode material and a preparation method thereof, a lithium ion battery and an electric device, and relates to the field of lithium ion batteries. The lithium iron phosphate positive electrode material comprises a core and a carbon coating layer coated on the surface of the core; the core contains doping elements M1 and M2, M1 is selected from at least one of Ti, Nb, V and W, and M2 is selected from at least one of Ti, V and Mg; the lithium iron phosphate positive electrode material comprises primary particles A, primary particles B and primary particles C; the D50 of the primary particles A, the primary particles B and the primary particles C increases in turn; and the average sphericity of the primary particles B and the primary particles C is 75-85%. The grading structure of the above-mentioned three levels of primary particles and the high sphericity of the primary particles B and the primary particles C make the lithium iron phosphate positive electrode material exhibit excellent powder stacking performance.

Description

Technical Field

[0001] This application relates to the field of lithium-ion batteries, and more particularly to a lithium iron phosphate cathode material and its preparation method, a lithium-ion battery, and an electrical device. Background Technology

[0002] Lithium iron phosphate (LiFePO4) cathode materials are widely used in power and energy storage batteries due to their high safety and long cycle life. Improving their volumetric energy density is one of the core directions of industrial development, and constructing particle size distribution to optimize powder packing density is a key technical path to achieve this goal.

[0003] For example, patent CN121180971A represents a typical two-stage gradation scheme, which involves physically mixing two lithium iron phosphate precursors with different particle sizes and iron sources, followed by a single co-sintering. While the intention of this scheme is clear, it has inherent drawbacks: the physical mixing process easily damages the carbon coating layer on the particle surface, and under the same sintering conditions, the shrinkage and densification behaviors of particles of different sizes and sources are difficult to coordinate. Small particles may be over-sintered, agglomerated, or even grown, while large particles have not yet completed sufficient densification, making it difficult to form an ideal packing structure.

[0004] In pursuit of higher packing density, three-stage gradation has become a new research direction. For example, patent CN120423517B divides the same raw material slurry into three parts, which are then independently ground, dried, and pre-sintered. The three pre-sintered materials with different particle sizes are then mixed and sintered a second time. While this method clearly achieves the preparation of three-stage particles, its process is essentially a physical combination of "separation followed by combination," requiring three repetitions of the drying and sintering process. This results in high energy consumption, low efficiency, and ultimately, reliance on physical mixing and co-sintering, failing to address the fundamental problem of poor interfacial integration between different pre-sintered material particles.

[0005] This reveals a contradiction in existing technologies: pursuing fine gradation (e.g., three-stage gradation) often comes at the cost of process simplicity and economy, while pursuing process simplification may be limited by the coarseness of the gradation structure. In existing technologies, interfacial bonding between particles is weak, limiting electrochemical performance. Their preparation methods all rely on physically mixing pre-prepared particles. In subsequent primary or secondary sintering, particles from different sources can only form limited point contacts or weak sintering necks, making it difficult to achieve strong chemical bonding or sufficient atomic diffusion. This weak interfacial bonding leads to increased electronic and ion transport impedance within the material, affecting rate performance, and during long-term cycling, interfacial detachment easily occurs due to volume expansion and contraction, resulting in accelerated capacity decay. Furthermore, existing technologies lack proactive and effective methods for optimizing the morphology of key particles. Their solutions mainly focus on generating particles with different sizes, but lack effective process intervention on how to improve the morphology of the particles themselves, especially making large particles that are prone to sharp edges and rough surfaces at high temperatures rounded and smooth. Rough and angular particles reduce powder flowability, limiting further increases in compaction density. Furthermore, sharp edges can pose safety hazards (such as puncturing the separator) and exacerbate side reactions during battery manufacturing and cycling. Current technologies do not provide a solution for simultaneously achieving particle surface rounding during gradation preparation.

[0006] Therefore, there is an urgent need in this field for an innovative solution that can overcome the above contradictions. It should be able to enhance the interfacial bonding between particles of different sizes to improve electrical performance without significantly increasing the complexity of the process; and be able to actively control and optimize the particle morphology (especially to improve sphericity), so as to achieve high compaction density while ensuring excellent processing performance, structural stability and long cycle life of the material. Summary of the Invention

[0007] The purpose of this application is to provide a lithium iron phosphate cathode material and its preparation method, lithium-ion battery and electrical device, to solve the problems of complex gradation process, rough particle morphology (especially large particles), many sharp edges and corners, and weak physical interface bonding between particles in existing lithium iron phosphate materials.

[0008] To achieve the above objectives, the first aspect of this application provides a lithium iron phosphate cathode material, comprising a core and a carbon coating layer covering the surface of the core; the core contains doping elements M1 and M2, wherein M1 is selected from at least one of Ti, Nb, V, and W, and M2 is selected from at least one of Ti, V, and Mg; The lithium iron phosphate cathode material includes primary particles A, primary particles B, and primary particles C; The D50 of the primary particle A, the primary particle B, and the primary particle C increases sequentially; The average sphericity of the primary particles B and C is 75-85%.

[0009] Optionally, the lithium iron phosphate cathode material satisfies at least one of the following conditions: (1) The D50 of the primary particle A is 0.05-0.4 μm; (2) The D50 of the primary particle B is 0.4-1 μm; (3) The D50 of the primary particle C is 1-3 μm; (4) The primary particle A contains dopant element M1, and the mass of element M1 accounts for 0.00015-0.003% of the total mass of the lithium iron phosphate cathode material; the primary particles B and C contain dopant element M2, and the mass of element M2 accounts for 0.0007-0.00425% of the total mass of the lithium iron phosphate cathode material; (5) The compaction density of the lithium iron phosphate cathode material is ≥2.6 g / cm³. 3 .

[0010] Optionally, the lithium iron phosphate cathode material satisfies at least one of the following conditions: (1) The mass of the primary particle A accounts for 15-30% of the total mass of the lithium iron phosphate cathode material; (2) The mass of the primary particles B and C accounts for 70-85% of the total mass of the lithium iron phosphate cathode material; (3) The carbon coating layer accounts for 0.5%-3% of the total mass of the lithium iron phosphate cathode material.

[0011] A second aspect of this application provides a method for preparing the lithium iron phosphate cathode material, comprising: The first slurry is obtained by first mixing and first grinding of iron phosphate doped with M1, first lithium source, carbon source A and water; the first slurry is then subjected to first spray drying to obtain first precursor microspheres; the first precursor microspheres are then subjected to first sintering under a protective atmosphere to obtain primary particles a. The iron phosphate, the second lithium source, carbon source B, water and additives are mixed and ground in a second process to obtain a second slurry; the second slurry is then spray-dried in a second process to obtain the second precursor microspheres. A portion of the second precursor microspheres is subjected to a second sintering to obtain primary particles b; another portion of the second precursor microspheres is subjected to a third sintering to obtain primary particles c; the endpoint temperature of the second sintering is lower than the endpoint temperature of the third sintering. The primary particles a and the repair slurry are mixed and ground for a third time to obtain a first mixture; the first mixture, the primary particles b and c are mixed and ground for a fourth time to obtain a second mixture; the second mixture is spray-dried and sintered for a third time to obtain a lithium iron phosphate cathode material. The repair slurry includes iron phosphate, a third lithium source, carbon source C, and phosphorus-containing sintering aids.

[0012] Optionally, the preparation method of lithium iron phosphate cathode material satisfies at least one of the following conditions: (1) The doping amount of M1 in the M1-doped iron phosphate is 3138-10460 ppm; (2) The first lithium source, the second lithium source and the third lithium source each independently include lithium carbonate and / or lithium hydroxide; (3) The carbon source A includes at least one of glucose, fructose and xylose; (4) The carbon source B includes at least one of sucrose, maltose and starch; (5) The additives include at least one of titanium dioxide, vanadium pentoxide and magnesium oxide; (6) The carbon source C comprises a composite carbon source consisting of carbohydrates and polyethylene glycol; the carbohydrates include sucrose and glucose; the mass ratio of the carbohydrates to the polyethylene glycol is 1-4:6-9; (7) The phosphorus-containing sintering aid includes at least one of phosphoric acid, lithium dihydrogen phosphate, ammonium dihydrogen phosphate, phosphate ester, pyrophosphate, and polyphosphoric acid.

[0013] Optionally, the preparation method of the lithium iron phosphate cathode material satisfies at least one of the following conditions: (1) The ratio of the molar amount of metal in the iron phosphate doped with M1 to the molar amount of lithium in the first lithium source is 1-1.05:1; (2) The ratio of the mass of the carbon source A to the mass of the iron phosphate doped with M1 is 4-6:100; (3) The solid content of the first slurry is 35-45%; (4) The D50 of the first slurry is ≤0.3μm; (5) The ratio of the molar amount of Fe in the iron phosphate to the molar amount of lithium in the second lithium source is 1-1.05:1; (6) The mass ratio of the carbon source B to the iron phosphate is 2.5-4:100; (7) The additive accounts for 1046-5230 ppm of the mass of the doping element in the second mixture, based on the mass of the doping element. (8) The solid content of the second slurry is 35-45%; (9) The D50 of the second slurry is 0.4-0.6 μm; (10) The mass ratio of the primary particles a, b and c in the fourth mixing is 20-35:35-60:15-30; (11) The molar amount of phosphorus in the phosphorus-containing sintering aid in the second mixture accounts for 0.2%-1% of the total molar amount of the primary particles a, b, and c; (12) The mass of the carbon source C in the second mixture accounts for 8-12% of the mass of the primary particle a; (13) The mass of ferric phosphate in the second mixture is 5-10% of the total mass of the primary particles a, b and c.

[0014] Optionally, the preparation method of the lithium iron phosphate cathode material satisfies at least one of the following conditions: (1) The inlet air temperature of the first spray dryer, the second spray dryer and the third spray dryer are each 200-220°C and the outlet air temperature is each 90-110°C. (2) The first sintering includes low-temperature sintering and high-temperature sintering performed sequentially; the heating rate of the low-temperature sintering is 3-5℃ / min, the final temperature is 350-450℃, and the holding time is 0.5-1h; the heating rate of the high-temperature sintering is 2-3℃ / min, the final temperature is 500-600℃, and the holding time is 8-12h. (3) The final temperature of the second sintering is 650-700℃, and the holding time is 5-8h; (4) The final temperature of the third sintering is 730-780℃, and the holding time is 6-9h; (5) The rotation speed of the third grinding is 900-1300 rpm; (6) The rotation speed of the fourth grinding is 200-600 rpm; (7) The fourth sintering includes a first-stage sintering, a second-stage sintering and a third-stage sintering performed sequentially; the final temperature of the first-stage sintering is 400-550℃ and the time is 0.5-1h; the final temperature of the second-stage sintering is 550-700℃ and the time is 5-7h; the final temperature of the third-stage sintering is 700-780℃ and the time is 2-4h.

[0015] A third aspect of this application provides a lithium-ion battery, including the aforementioned lithium iron phosphate cathode material.

[0016] A fourth aspect of this application provides an electrical device including the aforementioned lithium-ion battery.

[0017] Compared with the prior art, the beneficial effects of this application include: The lithium iron phosphate cathode material provided in this application uses primary particles C as the structural framework, primary particles B as the main contributor to capacity and fill the large gaps between primary particle C framework particles, while also connecting primary particles C and primary particles A; primary particles A serve as active functional units and microfillers, responsible for repairing and filling the surfaces of primary particles B and C; primary particles B and C have high sphericity and smooth surfaces; the gradation structure and high sphericity of the above three-level primary particles enable the lithium iron phosphate cathode material to exhibit excellent powder packing performance.

[0018] The method for preparing lithium iron phosphate cathode material provided in this application offers a "hierarchical construction-dissociation and recombination-chemical fusion" preparation method. Through differentiated first, second, and third sintering processes, three specific primary particle sizes of "structural units" (i.e., primary particle a, primary particle b, and primary particle c) are precisely constructed. Secondly, through designed third and fourth grinding steps, these "structural units" are dissociated, releasing the target primary particles, and then finely and uniformly mixed in the liquid phase with the repair raw materials (referring to raw materials in the repair slurry capable of generating lithium iron phosphate, including iron phosphate, a third lithium source, and carbon source C). Finally, utilizing the synergistic effect of phosphorus-containing sintering aids and carbon source C, the material is driven to migrate directionally in the fourth sintering process, achieving in-situ surface repair and rounding of large and medium particles (primary particles B and C), as well as interfacial chemical bonding between the three levels of particles. This simultaneously yields a product with high compaction density, excellent electrochemical performance, and high structural stability.

[0019] The lithium-ion battery and electrical equipment provided in this application have excellent electrochemical performance and good stability. Attached Figure Description

[0020] To more clearly illustrate the technical solutions of the embodiments of this application, the accompanying drawings used in the embodiments will be briefly described below. It should be understood that the following drawings only show some embodiments of this application and should not be regarded as a limitation on the scope of this application.

[0021] Figure 1 The image is a 5000x magnified SEM image of the lithium iron phosphate cathode material provided in Example 1. Figure 2 The image shown is a cross-sectional SEM image magnified 5000 times after ion cutting of the lithium iron phosphate cathode material provided in Example 1. Detailed Implementation

[0022] First, the solution provided in this application will be explained in more detail as follows: The first aspect of this application provides a lithium iron phosphate cathode material, including a core and a carbon coating layer covering the surface of the core; the core contains doping elements M1 and M2, wherein M1 is selected from at least one of Ti, Nb, V, and W, and M2 is selected from at least one of Ti, V, and Mg; Doping elements play a crucial role in lattice pinning and inhibiting grain growth; if the doping level is low, the inhibition effect is insufficient, and primary particles are prone to growth; if the doping level is high, impurity phases may be introduced, affecting electrochemical activity. The lithium iron phosphate cathode material includes primary particles A, primary particles B, and primary particles C; The D50 of the primary particle A, the primary particle B, and the primary particle C increases sequentially; The average sphericity of the primary particles B and C is 75-85%.

[0023] Optionally, the average sphericity of primary particles B and C can be any value between 75%, 80%, 85%, or 75-85%.

[0024] In some embodiments, the lithium iron phosphate cathode material satisfies at least one of the following conditions: (1) The D50 of the primary particle A is 0.05-0.4 μm; Optionally, the D50 of primary particle A can be any value between 0.05 μm, 0.1 μm, 0.15 μm, 0.2 μm, 0.25 μm, 0.3 μm, 0.35 μm, 0.4 μm, or 0.05-0.4 μm; (2) The D50 of the primary particle B is 0.4-1 μm; Optionally, the D50 of primary particle B can be any value between 0.4 μm, 0.5 μm, 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm, 1 μm or 0.4-1 μm; (3) The D50 of the primary particle C is 1-3 μm; Optionally, the D50 of the primary particle C can be any value between 1 μm, 1.5 μm, 2 μm, 2.5 μm, 3 μm, or 1-3 μm; (4) The primary particle A contains a dopant element M1, and the mass of element M1 accounts for 0.00015-0.003% of the total mass of the lithium iron phosphate cathode material; Optionally, the mass of element M1 in primary particle A can be any value between 0.00015, 0.0005, 0.001, 0.002, 0.003 or 0.00015-0.003 of the total mass of lithium iron phosphate cathode material; The mass of M2 element in the primary particles B and C accounts for 0.0007-0.00425% of the total mass of the lithium iron phosphate cathode material; Optionally, the mass of the M2 element in primary particles B and C can be any value between 0.0007, 0.001, 0.002, 0.003, 0.004, 0.00425 or 0.0007-0.00425 of the total mass of the lithium iron phosphate cathode material; (5) The compaction density of the lithium iron phosphate cathode material is ≥2.6 g / cm³. 3 .

[0025] Optionally, the compaction density of the lithium iron phosphate cathode material can be 2.6 g / cm³. 3 2.65 g / cm 3 2.67 g / cm 3 Or ≥2.6g / cm 3 Any value of .

[0026] In some embodiments, the lithium iron phosphate cathode material satisfies at least one of the following conditions: (1) The mass of the primary particle A accounts for 15-30% of the total mass of the lithium iron phosphate cathode material; Optionally, the mass of primary particle A can account for any value between 15%, 20%, 25%, 30% or 15-30% of the total mass of the lithium iron phosphate cathode material; (2) The mass of the primary particles B and C accounts for 70-85% of the total mass of the lithium iron phosphate cathode material; Optionally, the mass of primary particles B and C can account for any value between 70%, 75%, 80%, 85% or 70-85% of the total mass of the lithium iron phosphate cathode material; (3) The carbon coating layer accounts for 0.5%-3% of the total mass of the lithium iron phosphate cathode material.

[0027] Optionally, the carbon coating can account for any value between 0.5%, 1%, 1.5%, 2%, 2.5%, 3% or 0.5-3% of the total mass of the lithium iron phosphate cathode material.

[0028] A second aspect of this application provides a method for preparing the lithium iron phosphate cathode material, comprising: The first slurry is obtained by first mixing and first grinding of iron phosphate doped with M1, first lithium source, carbon source A and water; the first slurry is then subjected to first spray drying to obtain first precursor microspheres; the first precursor microspheres are then subjected to first sintering under a protective atmosphere to obtain primary particles a. In some embodiments, the preparation method provided in this application uses zirconia beads as the grinding medium to perform high-intensity sand milling. The high-energy grinding of the first grinding aims to break up the agglomeration of the precursor, create a basis for the generation of fine particles, and promote the uniform dispersion of the carbon source. The iron phosphate, the second lithium source, carbon source B, water and additives are mixed and ground in a second process to obtain a second slurry; the second slurry is then spray-dried in a second process to obtain the second precursor microspheres. A portion of the second precursor microspheres is subjected to a second sintering to obtain primary particles b; another portion of the second precursor microspheres is subjected to a third sintering to obtain primary particles c; the endpoint temperature of the second sintering is lower than the endpoint temperature of the third sintering. It is important to note that, without adding a pore-forming agent, by controlling the final temperatures of the second and third sintering processes (setting at least two different, specific high-temperature points), the precise differentiation of particle size is achieved with the simplest variables, taking advantage of the decisive influence of temperature on the grain growth (Ostwald ripening) rate. This allows the homologous precursor to spontaneously form primary particles b and c with different primary particle sizes under differentiated thermal processes. This step simplifies the process and lays the foundation for the three-stage primary particle gradation. The primary particles a and the repair slurry are mixed and ground for a third time to obtain a mixture; the first mixture, the primary particles b and c are mixed and ground for a fourth time to obtain a second mixture. It should be noted that the three primary particles b and c, which are completely dissociated after the fourth grinding, and the primary particles a with the repair material attached to the surface, are uniformly dispersed in the liquid phase containing phosphorus-containing sintering aid and carbon source C, achieving fine and uniform dispersion at the three-level primary particle scale in the liquid phase. The second mixture is subjected to a third spray drying and a fourth sintering to obtain lithium iron phosphate cathode material; It should be noted that due to epitaxial growth, some of the primary particles a, b, and c may melt together during the fourth sintering. This results in the primary particles A, B, and C after the final secondary sintering and crushing being slightly larger than the primary particles a, b, and c. Through the fourth sintering, the uniformly mixed tertiary particles are "fused" into a high-performance whole. The composite slurry includes iron phosphate, a third lithium source, carbon source C, and phosphorus-containing sintering aids.

[0029] It should be noted that the ferric phosphate in the composite slurry includes highly active ferric phosphate, which is amorphous or high specific surface area nano-ferric phosphate that can react in the fourth sintering. Phosphorus-containing sintering aids provide H +It gently etches all particle surfaces (especially primary particles a), increasing the surface hydroxyl density and reactive sites; it can also regulate the slurry pH and zeta potential, preventing particle agglomeration and ensuring microscopic uniformity; the phosphorus-containing sintering aid molecules are adsorbed on the primary particle surface and contact interface, and decompose during subsequent sintering, locally generating a P2O5 atmosphere that can accelerate material diffusion and drive the repair material to migrate to the surface of large and medium particles (edges and depressions) with the highest surface energy and grow epitaxially. It should also be noted that lithium iron phosphate cathode materials may contain trace amounts of residual phosphorus from phosphorus-containing liquid phase sintering aids. These residual phosphorus exist at the particle interface in the form of amorphous lithium phosphate or in combination with carbon. The molar content of phosphorus does not exceed 1% of the total molar amount of phosphorus in the lithium iron phosphate cathode material.

[0030] In some embodiments, the preparation method of lithium iron phosphate cathode material satisfies at least one of the following conditions: (1) The doping amount of M1 in the M1-doped iron phosphate is 3138-10460 ppm; Optionally, the doping amount of M1 in the M1-doped iron phosphate can be 3138 ppm, 5000 ppm, 10460 ppm or any value between 3138 and 10460 ppm; It is important to note that using pre-doped iron phosphate is beneficial for achieving atomic-level uniform doping. When sintering into lithium iron phosphate later, the doped elements can occupy the target lattice positions uniformly and stably, which more effectively hinders particle growth, thereby achieving the acquisition of small and uniform primary particles A. (2) The first lithium source, the second lithium source and the third lithium source each independently include lithium carbonate and / or lithium hydroxide; (3) The carbon source A includes at least one of glucose, fructose and xylose; It is important to note that carbon source A, as a low-temperature decomposition carbon source (initial decomposition temperature ~140℃), can form a uniform reducing atmosphere in the early stages of heating, ensuring Fe... 3+ Completely reduced to Fe 2+ Its early carbonization products can also physically isolate particles; (4) The carbon source B includes at least one of sucrose, maltose and starch; It should be noted that carbon source B should be selected from disaccharides or polysaccharides with higher decomposition temperatures, such as sucrose, which has a decomposition temperature range of 300~600℃. Using carbon source B avoids the drastic impact of premature or excessively strong reducing atmosphere on particle structure, and is more conducive to generating primary particles B and primary particles C of different sizes by controlling the sintering temperature. (5) The additives include at least one of titanium dioxide, vanadium pentoxide and magnesium oxide; It should be noted that the additives can ensure the electrochemical performance of primary particles B and C. (6) The carbon source C comprises a composite carbon source consisting of carbohydrates and polyethylene glycol; the carbohydrates include sucrose and glucose; the mass ratio of the carbohydrates to the polyethylene glycol is 1-4:6-9; Optionally, the mass ratio of carbohydrate compound to polyethylene glycol can be any value between 1:6, 2:6, 3:6, 4:6, 1:7, 1:8, 1:9, 4:9 or 1-4:6-9; It is important to note that the dual carbon source system in carbon source C helps to form a carbon layer with more uniform thickness and more complete coverage, reducing conductive blind spots; the polyethylene glycol (PEG) derived carbon structure binds more tightly to the matrix, which can effectively buffer the volume change stress during cycling and reduce the detachment of active materials from the conductive carbon layer; PEG promotes dispersion in the slurry and forms an adhesive transition layer in the early stage of carbonization; the saccharide compound provides the main carbon source; the two work together to form a continuous, dense and strongly adherent three-dimensional conductive carbon network between the particles that have completed epitaxial growth and have a tighter interfacial bond. (7) The phosphorus-containing sintering aid includes at least one of phosphoric acid, lithium dihydrogen phosphate, ammonium dihydrogen phosphate, phosphate ester, pyrophosphate, and polyphosphoric acid.

[0031] In some embodiments, the method for preparing the lithium iron phosphate cathode material satisfies at least one of the following conditions: (1) The ratio of the molar amount of metal in the iron phosphate doped with M1 to the molar amount of lithium in the first lithium source is 1-1.05:1; Optionally, the ratio of the molar amount of metal in the iron phosphate doped with M1 to the molar amount of lithium in the first lithium source can be 1:1, 1.01:1, 1.02:1, 1.03:1, 1.04:1, 1.05:1 or any value between 1 and 1.05:1; It should be noted that this slightly higher-than-stoichiometric amount of lithium can compensate for lithium volatilization during the sintering process and ensure complete reaction; a ratio below 1.00 may result in residual Fe2O3 and other impurity phases, reducing capacity; a ratio above 1.05 may generate inert phases such as Li3PO4. (2) The ratio of the mass of the carbon source A to the mass of the iron phosphate doped with M1 is 4-6:100; Optionally, the ratio of the mass of carbon source A to the mass of iron phosphate doped with M1 can be any value between 4:100, 5:100, 6:100 or 4:6:100. (3) The solid content of the first slurry is 35-45%; Optionally, the solid content of the first slurry can be any value between 35%, 40%, 45%, or 35-45%. (4) The D50 of the first slurry is ≤0.3μm; Optionally, the D50 of the first slurry can be any value of 0.1μm, 0.2μm, 0.3μm or ≤0.3μm; (5) The ratio of the molar amount of Fe in the iron phosphate to the molar amount of lithium in the second lithium source is 1-1.05:1; Optionally, the ratio of the molar amount of Fe in iron phosphate to the molar amount of lithium in the second lithium source can be 1:1, 1.01:1, 1.02:1, 1.03:1, 1.04:1, 1.05:1 or any value between 1 and 1.05:1; (6) The mass ratio of the carbon source B to the iron phosphate is 2.5-4:100; Optionally, the mass ratio of carbon source B to iron phosphate can be any value between 2.5:100, 3:100, 3.5:100, 4:100, or 2.5:4:100. (7) The additive accounts for 1046-5230 ppm of the mass of the doping element in the second mixture, based on the mass of the doping element. Optionally, the additive may be any value between 1046 ppm, 2000 ppm, 3000 ppm, 4000 ppm, 5230 ppm or 1046-5230 ppm based on the mass of the ferric phosphate in the second mixture; (8) The solid content of the second slurry is 35-45%; Optionally, the solid content of the second slurry can be any value between 35%, 40%, 45%, or 35-45%; (9) The D50 of the second slurry is 0.4-0.6 μm; Optionally, the D50 of the second slurry can be any value between 0.4 μm, 0.5 μm, 0.6 μm, or 0.4-0.6 μm; It should be noted that when the D50 of the second slurry is 0.4-0.6 μm, the second precursor microspheres obtained by spray drying can form a suitable porous structure during the subsequent third sintering. (10) The mass ratio of the primary particles a, b and the first primary particles in the fourth mixing is 20-35:35-60:15-30; Optionally, the mass ratio of primary particles a, primary particles b, and primary particles in the fourth mixing can be (20:35:15), (30:35:15), (35:35:15), (20:60:15), (20:35:30), (35:60:30), or any value between 20-35:35-60:15-30; It is important to note that primary particle C acts as the structural framework. If the proportion of primary particle C added is too low, the framework support will be weak and the compaction density will be low; if it is too high, the capacity will be reduced due to the large size and small specific surface area of ​​the large particles. Primary particle B is the main body of capacity and the connecting bridge, responsible for contributing the main capacity. Primary particle A acts as the active functional unit and microfiller, responsible for repairing and filling the surface of primary particle B and primary particle A. If the proportion of primary particle a added as an intermediate of primary particle A is too low, the repair and filling effect will be insignificant and the conductive network optimization will be insufficient; if it is too high, it may lead to excessively fine particles, affecting processing performance and potentially causing the overall particle size distribution to deviate from the optimal gradation. When the mass ratio of primary particle a, primary particle b, and primary particle c in the fourth mixture is 20-35:35-60:15-30, the optimal balance of packing density, active material ratio, and ion transport channels is achieved. (11) The molar amount of phosphorus in the phosphorus-containing sintering aid in the second mixture accounts for 0.2%-1% of the total molar amount of the primary particles a, b, and c; Optionally, the molar amount of phosphorus in the phosphorus-containing sintering aid in the second mixture is any value between 0.2%, 0.5%, 1% or 0.2%-1% of the total molar amount of primary particles a, b and c; Preferably, the molar amount of phosphorus in the phosphorus-containing sintering aid in the second mixture accounts for 0.3%-0.5% of the total molar amount of the primary particles a, b, and c; (12) The mass of the carbon source C in the second mixture accounts for 8-12% of the mass of the primary particle a; Optionally, the mass of carbon source C in the second mixture accounts for any value between 8%, 10%, 12% or 8-12% of the mass of primary particle a; (13) The mass of ferric phosphate in the second mixture is 5-10% of the total mass of the primary particles a, b and c.

[0032] Optionally, the mass of ferric phosphate in the second mixture can be any value between 5%, 6%, 7%, 8%, 9%, 10% or 5-10% of the total mass of primary particles a, b, and c.

[0033] In some embodiments, the method for preparing the lithium iron phosphate cathode material satisfies at least one of the following conditions: (1) The inlet air temperature of the first spray dryer, the second spray dryer and the third spray dryer are each 200-220°C and the outlet air temperature is each 90-110°C. Optionally, the inlet air temperature of the first spray dryer, the second spray dryer, and the third spray dryer can each be independently 200℃, 210℃, 220℃, or any value between 200℃ and 220℃, and the outlet air temperature can each be independently 90℃, 100℃, 110℃, or any value between 90℃ and 110℃. It should be noted that under the above spray drying conditions, precursor microspheres with good flowability are obtained; (2) The first sintering includes low-temperature sintering and high-temperature sintering performed sequentially; the heating rate of the low-temperature sintering is 3-5℃ / min, the final temperature is 350-450℃, and the holding time is 0.5-1h; the heating rate of the high-temperature sintering is 2-3℃ / min, the final temperature is 500-600℃, and the holding time is 8-12h. Optionally, the heating rate for low-temperature sintering can be any value between 3℃ / min, 4℃ / min, 5℃ / min, or 3-5℃ / min; the final temperature can be any value between 350℃, 400℃, 450℃, or 350-450℃; and the holding time can be any value between 0.5h, 0.6h, 0.7h, 0.8h, 0.9h, 1h, or 0.5-1h. For high-temperature sintering, the heating rate can be any value between 2℃ / min, 2.5℃ / min, 3℃ / min, or 2-3℃ / min; the final temperature can be any value between 500℃, 550℃, 600℃, or 500-600℃; and the holding time can be any value between 8h, 9h, 10h, 11h, 12h, or 8-12h. It is important to note the synergistic effect of metal element doping and low-temperature sintering: high-valence, small-radius metal ions can inhibit grain growth, which, combined with the physical isolation of the early carbon layer of carbon source A and the relatively low high-temperature sintering temperature (500-600℃), results in a triple inhibition of grain growth. At the same time, it maximizes the suppression of Ostwald ripening growth of grains, ultimately yielding small sintered spheres (primary particles a) composed of a large number of fine primary particles of 0.05-0.2μm. This unit has a high specific surface area and high surface energy, serving as a "active seed" for subsequent growth. If the temperature is too high (>600℃), the inhibition effect will be overcome by thermodynamic driving forces, and the particles will coarsen. (3) The final temperature of the second sintering is 650-700℃, and the holding time is 5-8h; Optionally, the final temperature of the second sintering can be any value between 650℃, 660℃, 670℃, 680℃, 690℃, 700℃ or 650-700℃, and the holding time can be any value between 5h, 6h, 7h, 8h or 5-8h. It should be noted that under the temperature conditions of the second sintering, carbon source B carbonization provides a reducing atmosphere, forming medium-sized sintered balls (primary particles b) formed by the agglomeration of primary particles of 0.4-0.8μm. The primary particle size is moderate, serving as the "main body" of the gradation system. (4) The final temperature of the third sintering is 730-780℃, and the holding time is 6-9h; Optionally, the final temperature of the third sintering can be any value between 730℃, 740℃, 750℃, 760℃, 770℃, 780℃ or 730-780℃, and the holding time can be any value between 6h, 7h, 8h, 9h or 6-9h. It should be noted that the third sintering has a higher temperature than the second sintering, which drives the primary particles to grow to 0.8-1.4μm, resulting in large sintered spheres (primary particles c) formed by the agglomeration of coarse primary particles. The primary particles are large in size and serve as the "skeleton" of the gradation. It should also be noted that by using the same precursor and carbon source, and by precisely controlling a single variable (final sintering temperature) to obtain two different sizes of particles (primary particles b and primary particles c), the carbon source B is selected as a disaccharide or polysaccharide with a higher decomposition temperature. This avoids the drastic impact of premature or excessively strong reducing atmosphere on the particle structure and is more conducive to generating primary particles B and primary particles C with different sizes by controlling the sintering temperature. (5) The rotation speed of the third grinding is 900-1300 rpm; Optionally, the rotation speed of the third grinding can be any value between 900 rpm, 1000 rpm, 1100 rpm, 1200 rpm, 1300 rpm, or 900-1300 rpm; It is important to note that the purpose of the third grinding is to break down the primary particles a, release the fine primary particles, and create conditions for their subsequent "repair" and "fusion"; to grind highly active raw materials such as iron phosphate to an extremely fine size and strongly adhere to the primary particles a and their surface; and to evenly disperse phosphorus-containing sintering aids, which can play a role in etching, activating particles, and stabilizing the slurry. (6) The rotation speed of the fourth grinding is 200-600 rpm; It should be noted that the fourth grinding is a low-intensity secondary grinding. This process mainly uses shear force and slight impact force to dissociate the two types of sintered balls (primary particles b and primary particles c), releasing the medium and large primary particles, while avoiding excessive crushing of the particles themselves. (7) The fourth sintering includes a first-stage sintering, a second-stage sintering and a third-stage sintering performed sequentially; the final temperature of the first-stage sintering is 400-550℃ and the time is 0.5-1h; the final temperature of the second-stage sintering is 550-700℃ and the time is 5-7h; the final temperature of the third-stage sintering is 700-780℃ and the time is 2-4h.

[0034] Optionally, the final temperature of the first-stage sintering can be any value between 400℃, 450℃, 500℃, 550℃, or 400-550℃, and the time can be any value between 0.5h, 0.6h, 0.7h, 0.8h, 0.9h, 1h, or 0.5-1h; the final temperature of the second-stage sintering can be any value between 550℃, 600℃, 650℃, 700℃, or 550-700℃, and the time can be any value between 5h, 6h, 7h, or 5-7h; the final temperature of the third-stage sintering can be any value between 700℃, 710℃, 720℃, 730℃, 740℃, 750℃, 760℃, 770℃, 780℃, or 700-780℃, and the time can be any value between 2h, 3h, 4h, or 2-4h.

[0035] It is important to note that in the first stage of sintering, the carbon source C decomposes within the temperature range of 400-550℃, and the phosphorus-containing sintering aid decomposes to produce P2O5, which activates the surface of the primary particles. In the second stage of sintering, within the temperature range of 550-700℃, the active atmosphere formed by P2O5 promotes material migration. At this time, the highly active "repair material" attached to the surface of primary particle a becomes the material source. Driven by surface energy, it preferentially migrates and undergoes heterogeneous nucleation and epitaxial growth on the surfaces of primary particles b and c (especially at high-energy edges and defects). The Gibbs-Thomson effect promotes the material to migrate from curved surfaces. The sharp corners with high curvature diffuse towards the planes with low curvature, achieving "in-situ polishing." These two mechanisms work together to continuously "polish" and "fill" the surface of the primary particles. This process continuously "repairs" the surfaces of primary particles b and c, making them rounded, thus achieving surface reconstruction and epitaxial growth of primary particles b and c. In the three-stage sintering, the carbon source C forms a continuous three-dimensional conductive network, tightly connecting the three levels of particles. Furthermore, the crystal structure of the epitaxial growth is more complete, and the carbon layer formed by the pyrolysis of the carbon source C is further graphitized. A robust three-dimensional electronic conductive network is constructed between the particles whose contact has been strengthened through epitaxial growth. If the temperature is below 700℃ in this stage, the degree of carbon graphitization is low, resulting in poor conductivity; if it is above 780℃, it may lead to over-sintering of the particles or morphological damage.

[0036] It should be noted that the lithium iron phosphate method provided in this application transforms the surfaces of primary particles b and c from rough and angular to smooth and rounded, improving the low sphericity of traditional processes to high sphericity, thereby enhancing powder flowability and compaction density (reaching 2.63 g / cm³ in the examples). 3 The bonding between primary particles has been upgraded from the traditional weak physical contact / sintering neck to a strong "chemical-physical" composite interface formed through epitaxial growth and carbon network. This significantly reduces the transport resistance of lithium ions and electrons across the particle interface. The tertiary particles are no longer a simple physical superposition, but constitute an organic whole in which "primary particle C opens up space, primary particle B fills and contributes, and primary particle A repairs and connects." Each component performs its own function and works together to form a strong interface bonding tertiary integrated structure through restorative growth and carbon network construction. Ultimately, this achieves a synergistic effect of high compaction, high capacity, and long lifespan, resulting in comprehensive optimization of key indicators such as compaction density, reversible capacity, first efficiency, rate capability, and cycle life of the final lithium iron phosphate cathode material, with no significant shortcomings.

[0037] This application simplifies the process through "homogeneous differentiation," simultaneously solves morphology and interface problems through "phosphorus-containing sintering aid-guided epitaxial growth," and achieves optimal system performance through "functionalized particle design and optimized gradation." These three technical approaches are interconnected and mutually reinforcing, generating a powerful synergistic effect. Ultimately, a lithium iron phosphate cathode material with high compaction, high sphericity, high interfacial strength, and excellent electrochemical performance was successfully prepared, perfectly addressing all the challenges posed by existing technologies.

[0038] It should also be noted that this application represents a paradigm shift from "physical mixing" to "chemical fusion," and its innovations are reflected in the following three aspects: 1. Innovation in process path: From "multi-line independent" to "homogeneous differentiation", efficient three-level gradation construction is achieved. It abandons the complex path of using multiple batches of independent raw materials, formulations and sintering to obtain particles of different sizes. It creatively adopts the strategy of "homogeneous precursor + temperature gradient differentiation". By utilizing the difference in intrinsic growth kinetics of a single undoped precursor at different sintering temperatures, primary particles b and c are directly obtained, which greatly simplifies the upstream process. Without sacrificing the gradation fineness, it significantly reduces the complexity and cost of the process and solves the industry contradiction that the pursuit of three-level gradation inevitably leads to complicated processes. 2. Innovation in morphology and interface control mechanisms: From "passive acceptance" to "active repair," achieving in-situ rounding and strong bonding; introducing phosphorus-containing sintering aids and designing "repair materials" to induce selective epitaxial growth on the surface during the fourth sintering process; the phosphorus-containing sintering aids create an active transport atmosphere, driving the repair material to migrate directionally to the defects on the surfaces of primary particles b and c, achieving "in-situ polishing" through the synergy of epitaxial growth and the Gibbs-Thomson effect; this application actively repairs rough particles into high sphericity morphology, while simultaneously constructing strong chemical bonding interfaces between different particles, thus simultaneously overcoming the two core challenges of poor morphology and weak interfaces; 3. Innovation in System Integration and Performance Synergy: From "simple superposition" to "integrated fusion," achieving a leap in performance synergy; integrating "homogeneous differentiation," "surface repair," and "functionalized particle design" (high doping of primary particle A, functional differentiation of primary particles B and C) with technical modules such as "liquid-phase dissociation mixing" and "PEG-carbohydrate dual-carbon source coating"; the modules are not simply connected in series, but deeply coupled. For example, the repair mechanism optimizes the morphology of graded particles, thereby releasing the density potential of tertiary stacking; while the enhanced interface and optimized carbon network ensure excellent charge transport capabilities even at high densities; Significant effect: It produced a synergistic effect of "1+1>2", enabling the material to achieve simultaneous and significant improvement in indicators that are usually mutually restrictive, such as compaction density, rate performance, and cycle life, resulting in a leap in comprehensive performance.

[0039] A third aspect of this application provides a lithium-ion battery, including the aforementioned lithium iron phosphate cathode material.

[0040] A fourth aspect of this application provides an electrical device including the aforementioned lithium-ion battery.

[0041] The implementation schemes of this application will be described in detail below with reference to specific embodiments. However, those skilled in the art will understand that the following embodiments are only for illustrating this application and should not be regarded as limiting the scope of this application. Unless otherwise specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments used without specified manufacturers are all conventional products that can be purchased commercially.

[0042] Example 1 The first aspect of this embodiment provides a lithium iron phosphate cathode material, including a core and a carbon coating layer covering the surface of the core; the mass of the carbon coating layer accounts for 1.2% of the total mass of the lithium iron phosphate cathode material; the lithium iron phosphate cathode material includes primary particles A, primary particles B and primary particles C; the mass of primary particle A accounts for 25% of the total mass of the lithium iron phosphate cathode material; the mass of primary particles B and primary particles C accounts for 75% of the total mass of the lithium iron phosphate cathode material; wherein, the relevant parameters of the lithium iron phosphate cathode material are shown in Table 1.

[0043] The second aspect of this embodiment provides a method for preparing lithium iron phosphate cathode material, the specific steps of which are as follows: S1: Preparation of primary particles a: Take 1000g of pre-doped titanium iron phosphate (titanium doping amount is 5230ppm), mix it with lithium carbonate (battery grade) and glucose (added amount is 5% of the mass of iron phosphate) measured according to Li / (Fe+Ti)=1.02, add deionized water to prepare slurry A with a solid content of 40%; use zirconia beads for sand milling to control the D50 of slurry A to 0.15μm, and then spray dry (inlet air 210℃, outlet air 100℃), and sinter in a nitrogen atmosphere: heat up to 400℃ at 5℃ / min and hold for 60 minutes, then heat up to 550℃ at 3℃ / min and hold for 10 hours, cool with furnace to obtain primary particles a with a size of 0.1-0.2μm; S2: Preparation of primary particles b and c: Take 2000g of undoped iron phosphate (Fe / P=0.965), mix it with lithium carbonate (battery grade), sucrose (added at 3.5% of the mass of iron phosphate), and TiO2 (calculated as Ti element, with Ti added at 3138ppm of the mass of iron phosphate) according to Li / Fe=1.02, and add deionized water to prepare slurry B with a solid content of 40%; mill to D50=0.5μm, and then spray dry; divide the dried precursor into two equal parts, and heat them at 680℃ for 6 hours in a nitrogen atmosphere to obtain primary particles b, and heat them at 750℃ for 7 hours to obtain primary particles c; the size of primary particle b is about 0.6μm, and the size of primary particle c is about 1.0μm; S3: Take 200g of the above-mentioned primary granules a and add the repair slurry, which includes ferric phosphate (amorphous, specific surface area > 10 m²). 230g of primary particles (a, b, and c), 7g of polyethylene glycol-4000 (PEG-4000), 23g of sucrose, and 2.5g of 85% phosphoric acid (approximately 0.4% of the total solid P molar weight, calculated as P) were mixed with deionized water and milled at 1200 rpm for 2 hours using 0.3mm zirconia beads to obtain slurry C (solid content 25%). 500g of primary particle b and 300g of primary particle c were added to the mixture and ground at low intensity at 400 rpm for 1 hour to obtain a uniform slurry D. The solid mass ratio of primary particles C, B, and A was approximately 30%:50%:20%. S4: Reconstruction sintering: The slurry D is spray-dried and heated to 500℃ in a nitrogen atmosphere at 5℃ / min and held for 60 minutes. Then, it is heated to 650℃ at 2℃ / min and held for 5 hours. Finally, it is heated to 750℃ at 1℃ / min and held for 4 hours. After sintering, it is pulverized by airflow and sieved to obtain the final lithium iron phosphate cathode material.

[0044] SEM image of the lithium iron phosphate cathode material magnified 5000 times, as shown below. Figure 1 As shown in the image, the cross-sectional SEM image after ion cutting is magnified 5000 times. Figure 2 As shown.

[0045] Example 2 The difference from Example 1 is that in step S1, pre-doped titanium iron phosphate is replaced with niobium-doped iron phosphate with the same doping amount; in step S2, TiO2 is replaced with V2O5, wherein the doping amount of V in V2O5 is the same as that of Ti in TiO2.

[0046] Example 3 The difference from Example 1 is that in step S1, pre-doped titanium iron phosphate is replaced with tungsten-doped iron phosphate with the same doping amount; in step S2, TiO2 is replaced with WO3, wherein the doping amount of W in WO3 is the same as that of Ti in TiO2.

[0047] Example 4 The difference from Example 1 is that in step S1, glucose is replaced with an equal mass of fructose; and in step S2, sucrose is replaced with an equal mass of maltose.

[0048] Comparative Example 1 The difference from Example 1 is that step S2 is not performed, and the products of step S2 (primary particles b and primary particles c) are not added in the subsequent step S3.

[0049] Comparative Example 2 The difference from Example 1 is that step S1 is not performed, and the product of step S2 (primary particle a) is not added in the subsequent step S3.

[0050] Comparative Example 3 The difference from Example 1 is that in step S2, the dried precursor is directly heated at 680°C for 6 hours in a nitrogen atmosphere to obtain primary particles b, that is, primary particles c are not prepared.

[0051] Comparative Example 4 The difference from Example 1 is that in step S2, the dried precursor is directly heated at 750°C for 7 hours in a nitrogen atmosphere to obtain primary particles c, that is, primary particles b are not prepared.

[0052] Comparative Example 5 The difference from Example 1 is that no repair slurry is added in step S3.

[0053] Comparative Example 6 The difference from Example 1 is that in step S3, the phosphorus-containing sintering aid (phosphoric acid) in the repair slurry is not added.

[0054] Comparative Example 7 The difference from Example 1 is that sucrose is not added to the repair slurry in step S3.

[0055] Comparative Example 8 The difference from Example 1 is that in step S1, the pre-doped titanium iron phosphate is replaced with an equal mass of iron phosphate, that is, the primary particle A is not doped.

[0056] Comparative Example 9 The difference from Example 1 is that TiO2 is not added in step S2, that is, primary particles B and C are not doped.

[0057] Comparative Example 10 The difference from Example 1 is that the carbon source in steps S1 and S2 is replaced.

[0058] The relevant product parameters of the lithium iron phosphate cathode materials prepared in the above embodiments and comparative examples are shown in Table 1. Among them, M percentage refers to the data of the doping elements in primary particles A, B and C as a percentage of the total mass of the lithium iron phosphate cathode material.

[0059] Table 1 Relevant Product Parameters

[0060] To further verify the physical and electrochemical properties of the lithium iron phosphate cathode materials provided in the above embodiments and comparative examples, sphericity, compaction density, tap density and electrochemical performance tests were conducted on them respectively. The specific test results are shown in Table 2.

[0061] Among them, the compaction density test was conducted under 3 tons of pressure using equipment from Sansi Zongheng.

[0062] The test method for sphericity is as follows: A cross-sectional scanning electron microscope image (e.g., 5000x magnification) of the lithium iron phosphate material after ion cutting is used. Figure 2 Import the data into the MIPAR software for analysis.

[0063] Test method for tapped density: Tested using a tapped density meter.

[0064] Electrochemical performance testing: Using lithium metal as the counter electrode, the lithium iron phosphate cathode materials provided in the above examples and comparative examples were tested in the voltage range of 2-3.75 V for 0.1C and 1C discharge specific capacity and capacity retention rate after 100 cycles of 1C discharge. Table 2 Performance Tests

[0065] analyze: The tests above show that the powder compaction density of the lithium iron phosphate cathode materials in Examples 1-4 is 2.57-2.63 g / cm³. 3 .

[0066] Compared with Comparative Examples 1-10, Examples 1-4 have higher 0.1C discharge capacity, 1C discharge capacity and / or higher compaction density, indicating that the examples have better overall performance.

[0067] Specifically, the compaction density of Comparative Example 1 is lower, which may be due to the lack of large and medium-sized particles that act as support. The overall electrochemical performance of Comparative Example 2 was poor, which may be due to the lack of small particles, insignificant repair and filling effects, and insufficient optimization of the conductive network. The compaction density of Comparative Example 3 was poor, which may be due to the lack of large particle skeletons that act as support. The compaction density and overall electrochemical performance of Comparative Example 4 were poor, which may be due to the lack of medium particles as a connecting bridge between the bulk capacity and the large-to-small particle size, and the high proportion of large particles.

[0068] The sphericity and compaction density of Comparative Example 5 are relatively low, which may be due to the lack of remediation materials, the presence of many sharp edges in the particles, and the difficulty in stacking them.

[0069] The sphericity and compaction density of Comparative Example 6 are relatively low, which may be due to the lack of repair phosphorus-containing additives, the inability of repair raw materials to migrate quickly during sintering, and the weak repair effect.

[0070] The overall electrochemical performance of Comparative Example 7 is poor, which may be due to the lack of a carbon coating layer and the poor electronic conductivity of the material. The compaction density of Comparative Example 8 is relatively low, which may be due to the fact that primary particle A is not doped, has a large particle size, low surface energy, and is difficult to carry repair materials to improve particle sphericity. The overall electrochemical performance of Comparative Example 9 is poor, which may be due to the lack of doping elements and the poor electronic and ionic conductivity of the material. The compaction density of Comparative Example 10 is relatively low, which may be due to the earlier decomposition temperature of carbon source B, the smaller size difference between primary particles B and C, and the failure to form a good tertiary gradation. Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of this application.

[0071] Furthermore, those skilled in the art will understand that although some embodiments herein include certain features included in other embodiments but not others, combinations of features from different embodiments are intended to be within the scope of this application and form different embodiments. For example, any of the claimed embodiments can be used in any combination. The information disclosed in this background section is intended only to enhance the understanding of the general background of this application and should not be construed as an admission or in any way implying that such information constitutes prior art known to those skilled in the art.

Claims

1. A lithium iron phosphate cathode material, characterized in that, It includes a core and a carbon coating layer covering the surface of the core; the core contains dopant elements M1 and M2, wherein M1 is selected from at least one of Ti, Nb, V, and W, and M2 is selected from at least one of Ti, V, and Mg; The lithium iron phosphate cathode material includes primary particles A, primary particles B, and primary particles C; The D50 of the primary particle A, the primary particle B, and the primary particle C increases sequentially; The average sphericity of the primary particles B and C is 75-85%.

2. The lithium iron phosphate cathode material according to claim 1, characterized in that, At least one of the following conditions must be met: (1) The D50 of the primary particle A is 0.05-0.4 μm; (2) The D50 of the primary particle B is 0.4-1 μm; (3) The D50 of the primary particle C is 1-3 μm; (4) The primary particle A contains dopant element M1, and the mass of element M1 accounts for 0.00015-0.003% of the total mass of the lithium iron phosphate cathode material; the primary particles B and C contain dopant element M2, and the mass of element M2 accounts for 0.0007-0.00425% of the total mass of the lithium iron phosphate cathode material. (5) The compaction density of the lithium iron phosphate cathode material is ≥2.6 g / cm³. 3 .

3. The lithium iron phosphate cathode material according to claim 1, characterized in that, At least one of the following conditions must be met: (1) The mass of the primary particle A accounts for 15-30% of the total mass of the lithium iron phosphate cathode material; (2) The mass of the primary particles B and C accounts for 70-85% of the total mass of the lithium iron phosphate cathode material; (3) The carbon coating layer accounts for 0.5%-3% of the total mass of the lithium iron phosphate cathode material.

4. A method for preparing the lithium iron phosphate cathode material according to any one of claims 1-3, characterized in that, include: The first slurry is obtained by first mixing and first grinding of iron phosphate doped with M1, first lithium source, carbon source A and water; the first slurry is then subjected to first spray drying to obtain first precursor microspheres. Under a protective atmosphere, the first precursor microspheres are subjected to a first sintering to obtain primary particles a; The iron phosphate, the second lithium source, carbon source B, water and additives are mixed and ground in a second process to obtain a second slurry; the second slurry is then spray-dried in a second process to obtain the second precursor microspheres. A portion of the second precursor microspheres is subjected to a second sintering to obtain primary particles b; another portion of the second precursor microspheres is subjected to a third sintering to obtain primary particles c; the endpoint temperature of the second sintering is lower than the endpoint temperature of the third sintering. The primary particles a and the repair slurry are mixed and ground for a third time to obtain a first mixture; the first mixture, the primary particles b and c are mixed and ground for a fourth time to obtain a second mixture; the second mixture is spray-dried and sintered for a third time to obtain a lithium iron phosphate cathode material. The repair slurry includes iron phosphate, a third lithium source, carbon source C, and phosphorus-containing sintering aids.

5. The method for preparing the lithium iron phosphate cathode material according to claim 4, characterized in that, At least one of the following conditions must be met: (1) The doping amount of M1 in the M1-doped iron phosphate is 3138-10460 ppm; (2) The first lithium source, the second lithium source and the third lithium source each independently include lithium carbonate and / or lithium hydroxide; (3) The carbon source A includes at least one of glucose, fructose and xylose; (4) The carbon source B includes at least one of sucrose, maltose and starch; (5) The additives include at least one of titanium dioxide, vanadium pentoxide and magnesium oxide; (6) The carbon source C comprises a composite carbon source consisting of carbohydrates and polyethylene glycol; the carbohydrates include sucrose and glucose; the mass ratio of the carbohydrates to the polyethylene glycol is 1-4:6-9; (7) The phosphorus-containing sintering aid includes at least one of phosphoric acid, lithium dihydrogen phosphate, ammonium dihydrogen phosphate, phosphate ester, pyrophosphate, and polyphosphoric acid.

6. The method for preparing the lithium iron phosphate cathode material according to claim 4, characterized in that, At least one of the following conditions must be met: (1) The ratio of the molar amount of metal in the iron phosphate doped with M1 to the molar amount of lithium in the first lithium source is 1-1.05:1; (2) The ratio of the mass of the carbon source A to the mass of the iron phosphate doped with M1 is 4-6:100; (3) The solid content of the first slurry is 35-45%; (4) The D50 of the first slurry is ≤0.3μm; (5) The ratio of the molar amount of Fe in the iron phosphate in the second mixture to the molar amount of lithium in the second lithium source is 1-1.05:1; (6) The mass ratio of the carbon source B to the iron phosphate is 2.5-4:100; (7) The additive, calculated as a doping element, accounts for 1046-5230 ppm of the mass of the iron phosphate in the second mixture; (8) The solid content of the second slurry is 35-45%; (9) The D50 of the second slurry is 0.4-0.6 μm; (10) The mass ratio of the primary particles a, b and c in the fourth mixing is 20-35:35-60:15-30; (11) The molar amount of phosphorus in the phosphorus-containing sintering aid in the second mixture accounts for 0.2%-1% of the total molar amount of the primary particles a, b, and c; (12) The mass of the carbon source C in the second mixture accounts for 8-12% of the mass of the primary particle a; (13) The mass of ferric phosphate in the second mixture is 5-10% of the total mass of the primary particles a, b and c.

7. The method for preparing the lithium iron phosphate cathode material according to any one of claims 4-6, characterized in that, At least one of the following conditions must be met: (1) The inlet air temperature of the first spray dryer, the second spray dryer and the third spray dryer are each 200-220°C and the outlet air temperature is each 90-110°C. (2) The first sintering includes low-temperature sintering and high-temperature sintering performed sequentially; the heating rate of the low-temperature sintering is 3-5℃ / min, the final temperature is 350-450℃, and the holding time is 0.5-1h; the heating rate of the high-temperature sintering is 2-3℃ / min, the final temperature is 500-600℃, and the holding time is 8-12h. (3) The final temperature of the second sintering is 650-700℃, and the holding time is 5-8h; (4) The final temperature of the third sintering is 730-780℃, and the holding time is 6-9h; (5) The rotation speed of the third grinding is 900-1300 rpm; (6) The rotation speed of the fourth grinding is 200-600 rpm; (7) The fourth sintering includes a first-stage sintering, a second-stage sintering and a third-stage sintering performed sequentially; the final temperature of the first-stage sintering is 400-550℃ and the time is 0.5-1h; the final temperature of the second-stage sintering is 550-700℃ and the time is 5-7h; the final temperature of the third-stage sintering is 700-780℃ and the time is 2-4h.

8. A lithium-ion battery, characterized in that, Including the lithium iron phosphate cathode material according to any one of claims 1-3.

9. An electrical appliance, characterized in that, Including the lithium-ion battery as described in claim 8.

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